Automatic retransmission request

ABSTRACT

A method for obtaining a report from a mobile station on the status of frames comprising an entire message transmitted to the mobile station is disclosed. First, a polling request is sent to the mobile station from a base station. A status report is then sent to the base station. The polling request specifies whether the mobile station should send the status report on a reservation basis (using a reserved frame) or on a contention basis (using an idle frame). The mobile station then transmits a bit map to the communication system to indicate which frames have been correctly received by the mobile station at the point when it received the polling request.

This application is a continuation in part of U.S. patent applicationSer. No. 08/147,254, entitled "A Method for Communicating in a WirelessCommunication System," which was filed on Nov. 1, 1993, and which isincorporated in this application by reference.

BACKGROUND

The present invention relates to a method for transmitting messagesbetween mobile stations and a central switching system, and moreparticularly to transmitting these messages using a more efficientcommunications link protocol over the air-interface of a cellulartelephone system.

In a typical cellular radio system, a geographical area, e.g., ametropolitan area, is divided into several smaller, contiguous radiocoverage areas called "cells." The cells are served by a series of fixedradio stations called "base stations." The base stations are connectedto and controlled by a mobile services switching center (MSC). The MSC,in turn, is connected to the landline (wireline) public switchedtelephone network (PSTN). The telephone users (mobile subscribers) inthe cellular radio system are provided with portable (hand-held),transportable (hand-carried) or mobile (car-mounted) telephone units(mobile stations) which communicate voice and/or data with the MSCthrough a nearby base station. The MSC switches calls between and amongwireline and mobile subscribers, controls signalling to the mobilestations, compiles billing statistics, and provides for the operation,maintenance and testing of the system.

FIG. 1 illustrates the architecture of a conventional cellular radiosystem built according to the Advanced Mobile Phone Service (AMPS)standard. In FIG. 1, an arbitrary geographic area may be seen dividedinto a plurality of contiguous radio coverage areas, or cells, C1-C10.While the system of FIG. 1 is, for illustration purposes, shown toinclude only ten cells, the number of cells may be much larger inpractice. Associated with and located in each of the cells C1-C10 is abase station designated as a corresponding one of a plurality of basestations B1-B10. Each of the base stations B1-B10 includes a pluralityof channel units, each comprising a transmitter, a receiver and acontroller, as is well known in the art.

In FIG. 1, the base stations B1-B10 are located at the center of thecells C1-C10, respectively, and are equipped with omni-directionalantennas transmitting equally in all directions. In this case, all thechannel units in each of the base stations B1-B10 are connected to oneantenna. However, in other configurations of the cellular radio system,the base stations B1-B10 may be located near the periphery, or otherwiseaway from the centers of the cells C1-C10 and may illuminate the cellsC1-C10 with radio signals directionally. For example, the base stationmay be equipped with three directional antennas, each one covering a 120degrees sector cell as shown in FIG. 2. In this case, some channel unitswill be connected to one antenna covering one sector cell, other channelunits will be connected to another antenna covering another sector cell,and the remaining channel units will be connected to the remainingantenna coveting the remaining sector cell. In FIG. 2, therefore, thebase station serves three sector cells. However, it is not alwaysnecessary for three sector cells to exist and only one sector cell needsto be used to cover, for example, a road or a highway.

Returning to FIG. 1, each of the base stations B1-B10 is connected byvoice and data links to a mobile switching center (MSC) 20 which is, inturn, connected to a central office (not shown) in the public switchingtelephone network (PSTN), or a similar facility, e.g., an integratedsystem digital network (ISDN). The relevant connections and transmissionmodes between the mobile switching center MSC 20 and the base stationsB1-B10, or between the mobile switching center MSC 20 and the PSTN orISDN, are well known to those of ordinary skill in the art and mayinclude twisted wire pairs, coaxial cables, fiber optic cables ormicrowave radio channels operating in either analog or digital mode.Further, the voice and data links may either be provided by the operatoror leased from a telephone company (telco).

With continuing reference to FIG. 1, a plurality of mobile stationsM1-M10 may be found within the cells C1-C10. Again, while only tenmobile stations are shown in FIG. 1, the actual number of mobilestations may be much larger in practice and will generally exceed thenumber of base stations. Moreover, while none of the mobile stationsM1-M10 may be found in some of the cells C1-C10, the presence or absenceof the mobile stations M1-M10 in any particular one of the cells C1-C10depends on the individual desires of each of the mobile subscribers whomay travel from one location in a cell to another or from one cell to anadjacent or neighboring cell.

Each of the mobile stations M1-M10 includes a transmitter, a receiver, acontroller and a user interface, e.g., a telephone handset, as is wellknown in the art. Each of the mobile stations M1-M10 is assigned amobile identification number (MIN) which, in the United States, is adigital representation of the telephone directory number of the mobilesubscriber. The MIN defines the subscription of the mobile subscriber onthe radio path and is sent from the mobile station to the MSC 20 at callorigination and from the MSC 20 to the mobile station at calltermination. Each of the mobile stations M1-M10 is also identified by anelectronic serial number (ESN) which is a factory-set, "unchangeable"number designed to protect against the unauthorized use of the mobilestation. At call origination, for example, the mobile station will sendthe ESN to the MSC 20. The MSC 20 will compare the received ESN to a"blacklist" of the ESNs of mobile stations which have been reported tobe stolen. If a match is found, the stolen mobile station will be deniedaccess.

Each of the cells C1-C10 is allocated a subset of the radio frequency(RF) channels assigned to the entire cellular system by the concernedgovernment authority, e.g., the Federal Communications Commission (FCC)in the United States. Each subset of RF channels is divided into severalvoice or speech channels which are used to carry voice conversations,and at least one paging/access or control channel which is used to carrysupervisory data messages, between each of the base stations B1-B10 andthe mobile stations M1-M10 in its coverage area. Each RF channelcomprises a duplex channel (bidirectional radio transmission path)between the base station and the mobile station. The RF channel consistsof a pair of separate frequencies, one for transmission by the basestation (reception by the mobile station) and one for transmission bythe mobile station (reception by the base station). Each channel unit inthe base stations B1-B10 normally operates on a preselected one of theradio channels allocated to the corresponding cell, i.e., thetransmitter (TX) and receiver (RX) of the channel unit are tuned to apair of transmit and receive frequencies, respectively, which does notchange. The transceiver (TX/RX) of each mobile station M1-M10, however,may tune to any of the radio channels specified in the system.

In typical land line systems, remote stations and control centers areconnected by copper or fiber optic circuits which have a data throughputcapacity and performance integrity that is generally significantlybetter than the data throughput capacity and performance integrityprovided by an air interface in a cellular telephone system. As aresult, the conciseness of overhead required to manage any selectedcommunication link protocol for land line systems is of secondaryimportance. In cellular telephone systems, an air interfacecommunications link protocol is required in order to allow a mobilestation to communicate with a cellular switching system. Acommunications link protocol is used to initiate and to receive cellulartelephone calls.

The electromagnetic spectrum available for use by cellular telephonesystems is limited and is divided into units called channels. Individualchannels are used as communication links either on a shared basis or ona dedicated or reserved basis. When individual channels are used ascommunication links on a shared basis, multiple mobile stations mayeither listen to or contend for the same channels. In the contendingsituation, each shared channel can be used by a plurality of mobilestations which compete to obtain exclusive use of the channel for alimited period of time. On the other hand, when individual channels areused as communication links on a dedicated basis, a single mobilestation is assigned the exclusive use of the channel for as long as itneeds it.

In light of the generally reduced data throughput capacity andperformance integrity afforded by an individual channel in a channelsharing situation in a cellular telephone environment, the selection ofan efficient air interface protocol to serve as the basis of thecommunication link becomes paramount.

The communication link protocol is commonly referred to as a layer 2protocol within the communications industry and its functionalityincludes the delimiting or framing of higher level messages. Traditionallayer 2 protocol framing mechanisms of bit stuffing and flag charactersare commonly used in land line networks today to frame higher layermessages, which are referred to as layer 3 messages. These layer 3messages may be sent between communicating layer 3 peer entitiesresiding within mobile stations and cellular switching systems.

In cellular systems, the likelihood of successfully sending a messageover a radio channel is inversely proportional to the length of themessage since the entire message will be considered to be in error evenif only a single bit of the transmitted message is received in error. Inorder to address this problem, messages are first divided into smallpackets or frames. Thus, it becomes important for the cellular system toknow if all of the transmitted packets are correctly received by amobile station.

SUMMARY

According to the present invention, in order to make this determinationa base station may send a mobile station any given number of framesusing automatic retransmission request (ARQ) wherein the base stationmay ask the mobile station to send a current status report on the framesit has received and then resend any frames not received correctly. Forexample, the mobile station can be asked to identify what frames it hasreceived at any point during an ARQ based transmission or the mobilestation can autonomously send a message to the base station stating whatframes it has received.

According to one embodiment of the present invention, a method forobtaining a report from a mobile station on the status of framescomprising an entire message transmitted to the mobile station isdisclosed. First, a status request is sent to the mobile station from abase station. A status report is then sent to the base station. Thestatus request specifies whether the mobile station should send thestatus report on a reservation basis (i.e., using a reserved frame) oron a contention basis (i.e., using an idle frame). The mobile stationthen transmits a bit map to the communication system to indicate whichframes have been correctly received by the mobile station at the pointin time when it received the status request.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail withreference to preferred embodiments of the invention, given only by wayof example, and illustrated in the accompanying drawings, in which:

FIG. 1 illustrates the architecture of a conventional cellular radiosystem;

FIG. 2 illustrates a three sector cell which may be used in the systemshown in FIG. 1;

FIG. 3 illustrates a block diagram of an exemplary cellular mobile radiotelephone system;

FIG. 4 illustrates the logical channels which make up the digitalcontrol channel according to one embodiment of the present invention;

FIGS. 5a-b illustrate SPACH Header sections A and B, respectively,according to one embodiment of the present invention;

FIGS. 6a-b illustrate the Random Access Procedures for a base stationand a mobile station according to one embodiment of the presentinvention;

FIGS. 7a-b illustrate a SPACH ARQ Mode Procedure for a mobile stationand a base station according to one embodiment of the present invention;

FIG. 8 illustrates an ARQ Mode Begin frame according to one embodimentof the present invention; and

FIG. 9 illustrates an ARQ Mode Continue frame according to oneembodiment of the present invention.

DETAILED DESCRIPTION

Although the description hereinafter focuses on systems which complywith IS-54B and its successors, the principles of the present inventionare equally applicable to a variety of wireless communication systems,e.g., cellular and satellite radio systems, irrespective of theparticular mode of operation (analog, digital, dual-mode, etc.), theaccess technique (FDMA, TDMA, CDMA, hybrid FDMA/TDMA/CDMA,etc.), or thearchitecture (macrocells, microcells, picocells, etc.). As will beappreciated by one skilled in the art, the logical channel which carriesspeech and/or data may be implemented in different ways at the physicallayer level. The physical channel may be, for example, a relativelynarrow RF band (FDMA), a time slot on a radio frequency (TDMA), a uniquecode sequence (CDMA), or a combination of the foregoing. For purposes ofthe present invention, the term "channel" means any physical channelwhich can carry speech and/or data, and is not limited to any particularmode of operation, access technique or system architecture.

This application contains subject matter which is related to co-pendingU.S. patent application Ser. No. 07/955,591, entitled "Method andApparatus for Communication Control in a Radiotelephone System," filedon Oct. 2, 1992, to co-pending U.S. patent application Ser. No.07/956,640, entitled "Digital Control Channel," filed on Oct. 5, 1992,to co-pending U.S. patent application Ser. No. 08/047,452, entitled"Layer 2 Protocol for the Random Access Channel and the Access ResponseChannel," filed on Apr. 19, 1993, to co-pending U.S. patent applicationSer. No. 08/147,254, entitled "A Method For Communicating in a WirelessCommunication System," filed on Nov. 1, 1993, to co-pending U.S. patentapplication Ser. No. 07/967,027, entitled "Multi-Mode SignalProcessing," filed on Oct. 27, 1992, and to co-pending U.S. patentapplication Ser. No. 08/140,467, entitled "A Method of Effecting RandomAccess in a Mobile Radio System," filed on Oct. 25, 1993. These sixco-pending applications are incorporated herein by reference.

FIG. 3 represents a block diagram of an exemplary cellular mobileradiotelephone system according to one embodiment of the presentinvention. The system shows an exemplary base station 110 and a mobilestation 120. The base station includes a control and processing unit 130which is connected to the mobile switching center MSC 140 which in turnis connected to the public switched telephone network (not illustrated).

The base station 110 for a cell includes a plurality of voice channelshandled by voice channel transceiver 150 which is controlled by thecontrol and processing unit 130. Also, each base station includes acontrol channel transceiver 160 which may be capable of handling morethan one control channel. The control channel transceiver 160 iscontrolled by the control and processing unit 130. The control channeltransceiver 160 broadcasts control information over the control channelof the base station or cell to mobiles locked to that control channel.

When the mobile 120 is in an idle mode, the mobile periodically scansthe control channels of base stations like base station 110 to determinewhich cell to lock on or camp to. The mobile 120 receives the absoluteand relative information broadcast on a control channel at its voice andcontrol channel transceiver 170. Then, the processing unit 180 evaluatesthe received control channel information which includes thecharacteristics of the candidate cells and determines which cell themobile should lock onto. The received control channel information notonly includes absolute information concerning the cell with which it isassociated, but also contains relative information concerning othercells proximate to the cell which the control channel is associated.

For a better understanding of the structure and operation of the presentinvention, the digital control channel (DCC) may be divided into threelayers: layer 1 (the physical layer), layer 2 and layer 3. The physicallayer defines the parameters of the physical communications channel,e.g., RF spacing, modulation characteristics, etc. Layer 2 (L2) definesthe techniques necessary for the accurate transmission of informationwithin the constraints of the physical channel, e.g., error correctionand detection, etc. Layer 3 (L3) defines the procedures for receptionand processing of information transmitted over the physical channel.

According to the present invention, the DCC is comprised of the logicalchannels shown in FIG. 4. The DCC logical channels include: thebroadcast control channel (BCCH) (which is further comprised of the fastbroadcast control channel F-BCCH, the extended broadcast control channelE-BCCH, and the short message service broadcast control channel S-BCCH);the SPACH which includes a short message service point to point channel(SMSCH), a paging channel (PCH) and an access response channel (ARCH);and a random access control channel (RACH).

The BCCH acronym is used to refer collectively to the F-BCCH, E-BCCH,and S-BCCH logical channels. These three logical channels are used, ingeneral, to carry generic, system-related information. Some attributesof these three channels are: unidirectional (i.e., downlink), shared,point-to-multipoint (i.e., broadcast), and unacknowledged. The fast BCCHis a logical channel used to broadcast, for example, time criticalsystem information. The extended BCCH is a logical channel used tobroadcast, for example, system information that is less time criticalthan the information sent on the F-BCCH. The short message service BCCHis a logical channel that is used to broadcast, for example, shortmessages used for the SMS broadcast service.

The SPACH channel is a logical channel that includes the SMSCH, PCH andARCH and is used to send information to specific mobile stations for SMSpoint-to-point, paging and access response purposes. The paging channelPCH is a subset of the SPACH used to deliver pages and orders. Theaccess response channel ARCH is a subset of the SPACH to which themobile station may autonomously move upon successful completion of anaccess on a random access channel. The ARCH may be used to convey analogvoice channel or digital traffic channel assignments or other responsesto a mobile access attempt. Layer 2 automatic retransmission requestsare possible for the ARCH and SMSCH using acknowledgement flames sent onthe RACH. The SMS point-to-point channel SMSCH is used to deliver shortmessages to a specific mobile station receiving SMS services. Theattributes of the SPACH are: unidirectional (downlink) and shared. ThePCH is point-to-multipoint and unacknowledged. The ARCH and SMSCH arepoint-to-point and may be acknowledged or unacknowledged.

The random access channel RACH is used to request access to the system.The attributes of this channel are unidirectional (uplink), shared,point-to-point, and acknowledged. Contention resolution and/or collisionavoidance information is provided on the forward subchannelcorresponding to any given frame sent on the RACH.

The SPACH layer 2 protocol can be used whenever a TDMA burst is used tocarry point-to-point SMS, Paging, or ARCH information. A single SPACHlayer 2 protocol frame can be constructed so as to fit, for example,within a 125 bit envelope. Additional bits (e.g., five) are reserved foruse as tail bits resulting in a total of 130 bits of information carriedwithin each slot assigned for SPACH purposes. A summary of the possibleSPACH formats is provided in the first table below. A summary of thefields comprising layer 2 protocol frames for SPACH operation isprovided in the second table below.

Similar frame formats can be used for the SPACH channels such that theframes will have a common Header A. The contents of the Header Adetermine whether or not additional information, known as Header B, ispresent in any given SPACH frame. The Header A discriminates betweenhard (dedicated) page frames, PCH frames, ARCH frames and SMSCH frames.A Hard Triple Page frame containing three 34-bit mobile stationidentifications MSIDs can be sent on the PCH (Burst Usage BU=Hard TriplePage). A Hard Quadruple Page frame containing four 20 bit or 24 bitMSIDs can also be sent on the PCH (BU=Hard Quadruple Page).

One or more L3 messages may be transmitted in one frame, or continuedover many frames. MSIDs are only carried within frames where BU=Hard,Triple Page, Hard Quadruple Page, and ARQ Mode BEGIN in addition toBU=PCH, ARCH and SMSCH where the Burst Type BT may be set to SingleMSID, Double MSID, Triple MSID or Quadruple MSID. The mobile stationidentity type (IDT) field identifies the format of all MSIDs carriedwithin a given SPACH frame (i.e., no mixing of MSID formats is allowed).Pages carried on the PCH are not allowed to continue beyond a singleSPACH frame (even though the protocol allows for it). All other PCHmessages may continue beyond a single SPACH frame.

For non ARQ mode operation, the L2 SPACH protocol supports sending asingle L3 message to multiple MSIDs in addition to the fixed one-to-onerelationship between MSIDs and L3 messages. The Message Mapping field(MM) is used to control this aspect of the layer 2 frame operation. Avalid SPACH frame requires that all L2 header information pertinent to agiven L2 frame be included entirely within that frame, i.e., L2 headerfrom a given SPACH frame cannot wrap into another SPACH frame. TheOffset Indicator field (OI) is used to allow both the completion of apreviously started layer 3 message and the start of a new layer 3message to occur within a single SPACH frame.

The following table summarizes possible SPACH formats:

    ______________________________________                                                                         CAN BE                                                    SMS  PCH    ARCH    CONTINUED                                    ______________________________________                                        Single MSID    Y      Y      Y     Y                                          Double MSID    N      Y      Y     Y                                          Triple MSID    N      Y      Y     Y                                          Quadruple MSID N      Y      Y     Y                                          Hard Triple Page (MIN)                                                                       N      Y      N     N                                          Hard Quadruple Page                                                                          N      Y      N     N                                          (TMSI)                                                                        Continue       Y      Y      Y     Y                                          ARQ Mode BEGIN Y      N      Y     Y                                          ARQ Mode CONTINUE                                                                            Y      N      Y     Y                                          ______________________________________                                    

FIG. 5a illustrates the SPACH Header A according to one embodiment ofthe present invention. The SPACH Header A contains burst usageinformation and flags for managing mobile stations in a sleep mode. TheBU field provides a high level indication of burst usage. According tothe present invention, the operation performed on each SPACH channel isnot predetermined. The BU field indicates whether the burst is beingused for paging, access response, or short message services. The flagsindicate changes in sleep mode configuration as well as broadcastcontrol channel information. This header can be present in all possibleSPACH frame types. FIG. 5b illustrates the SPACH Header B according toone embodiment of the present invention. The SPACH Header B containssupplementary header information used to identify the remaining contentof the layer 2 frame. This header is present when Header A indicates aburst usage of type PCH, ARCH or SMSCH.

The following table summarizes the SPACH Layer 2 Protocol fields:

    __________________________________________________________________________    Field Name        Length (bits)                                                                        Values                                               __________________________________________________________________________    BU = Burst Usage  3      000 = Hard Triple Page (34 bit                                                MSID)                                                                         001 = Hard Quad Page (20 or 24                                                bit MSID)                                                                     010 = PCH Burst                                                               011 = ARCH Burst                                                              100 = SMSCH Burst                                                             101 = Reserved                                                                110 = Reserved                                                                111 = Null                                           PCON = PCH Continuation                                                                         1      0 = No PCH Continuation                                                       1 = PCH Continuation,                                                         Activated                                            BCN = BCCH Change Notification                                                                  1      Transitions whenever there is a                                               change in F-BCCH information.                        SMSN = SMS Notification                                                                         1      Transitions whenever there is a                                               change in S-BCCH information.                        PFM = Paging Frame Modifier                                                                     1      0 = Use assigned PF                                                           1 = Use one higher than assigned                                              PF                                                   BT = Burst Type   3      000 = Single MSID Frame                                                       001 = Double MSID Frame                                                       010 = Triple MSID Frame                                                       011 = Quadruple MSID Frame                                                    100 = Continue Frame                                                          101 = ARQ Mode Begin                                                          110 = ARQ Mode Continue                                                       111 = Reserved                                       IDT = Identity Type                                                                             2      00 = 20 bit TMSI                                                              01 = 24 bit MINI per IS-54B                                                   10 = 34 bit MIN per IS-54B                                                    11 = 50 bit IMSI                                     MSID = Mobile Station Identity                                                                  20/24/34/50                                                                          20 bit TMSI                                                                   24 bit MINI                                                                   34 bit MIN                                                                    50 bit IMSI                                          MM = Message Mapping                                                                            1      0 = One instance of L3LI and                                                  L3DATA per instance of MSID.                                                  1 = One instance of L3LI and                                                  L3DATA for multiple MSIDS.                           OI = Offset Indicator                                                                           1      0 = No message offset included.                                               1 = Message offset included.                         CLI = Continuation Length                                                                       7      Number of bits remaining in the                      Indicator                previous L3 message.                                 L3LI = Layer 3 Length Indicator                                                                 8      Variable length layer 3 messages                                              supported up to a maximum of 255                                              octets.                                              L3DATA = Layer 3 Data                                                                           Variable                                                                             Contains a portion (some or all) of                                           the layer 3 message having an                                                 overall length as indicated by L3LI.                                          The portion of this field not used to                                         carry layer 3 information is filled                                           with zeros.                                          PE = Partial Echo 7      The 7 least significant bits of the                                           mobile station IS-54B MIN.                           TID = Transaction Identity                                                                      2      Indicates which ARQ mode                                                      transaction is being transmitted on                                           the ARCH or SMSCH.                                   FRNO = Frame Number                                                                             5      Uniquely identifies specific frames                                           sent in support of an ARQ mode                                                transaction.                                         FILLER = Burst Filler                                                                           Variable                                                                             All filler bits are set zero.                        CRC = Cyclic Redundancy Code                                                                    16     Same Generator polynomial as IS-                                              54B (includes DVCC)                                  __________________________________________________________________________

According to the present invention, a mobile station (MS) can be in aplurality of states. A mobile station shall be in the "start randomaccess" state before the first unit of a message that is to betransmitted by a random access has been transmitted. The mobile stationshall be in the "start reserved access" state before the first unit of amessage that is to be transmitted by a reservation based access has beentransmitted. The mobile station shall be in the "more units" state ifthere are more units associated with the current access event yet to betransmitted. The mobile station shall be in the "after last burst" stateif the last unit of an access event has been transmitted. Finally, themobile station shall be in the "success" state after a complete messagehas been sent successfully.

The layer 2 protocol is also controlled by a plurality of flags. Forwardshared channel feedback (SCF) flags are used to control transmissions onthe RACH. A busy/reserved/idle (BRI) flag is used to indicate whetherits corresponding uplink RACH slot is Busy, Reserved or Idle. Six bitsare used for these flags and the different conditions are encoded asshown in the table below:

    ______________________________________                                               BRI.sub.5                                                                          BRI.sub.4                                                                              BRI.sub.3                                                                            BRI.sub.2                                                                            BRI.sub.1                                                                          BRI.sub.0                             ______________________________________                                        Busy     1      1        1    1      0    0                                   Reserved 0      0        1    1      1    1                                   Idle     0      0        0    0      0    0                                   ______________________________________                                    

A received/not received (R/N) flag is used to indicate whether or notthe base station received the last transmitted burst. A five timesrepetition code is used for encoding this flag as shown in the tablebelow:

    ______________________________________                                                R/N.sub.4                                                                           R/N.sub.3                                                                              R/N.sub.2                                                                             R/N.sub.1                                                                            R/N.sub.0                               ______________________________________                                        Received  1       1        1     1      1                                     Not Received                                                                            0       0        0     0      0                                     ______________________________________                                    

According to the present invention, partial echo (PE) information isused to identify which MS was correctly received after the initial burstof random access has been sent or which MS is intended to have access toa reserved RACH slot. For example, the seven least significant bits LSBsof an IS-54B MIN can be used as partial echo information.

The following table shows how a mobile station decodes received flagsaccording to its Layer 2 state. Note that only the flags relevant to itsLayer 2 state are shown. In the "Start random access" state, the BRIflag is the only relevant flag. During a multiburst message transmissionboth the BRI and R/N flags are relevant. In the summations in thefollowing table, b_(i) equals the bit value.

    __________________________________________________________________________    Layer 2 State                                                                         Busy/Reserved/ldle                                                                              Received/Not Received                               __________________________________________________________________________            Busy  Reserved                                                                            Idle  Received                                                                            Not                                                                           received                                              111100                                                                              001111                                                                              000000                                                                              11111 00000                                         Start random access                                                                    ##STR1##         N/A   N/A                                           Start reserved                                                                        Reserved IF < 3 bits difference to                                                              N/A   N/A                                           access  Reserved flag code value                                              More units                                                                            Busy IF < 4 bits difference to Busy flag code value                                              ##STR2##                                                                            ##STR3##                                     After last burst                                                                      Busy IF < 4 bits difference to Busy flag code value                                              ##STR4##                                                                            ##STR5##                                     __________________________________________________________________________

The mobile station interprets a received coded partial echo (CPE) valueas having been correctly decoded if it differs by less than 3 bits fromthe correctly coded partial echo. This is referred to as PE match.

A mobile station is allowed a maximum of Y+1, where Y=(0 . . . 7),access attempts before considering the attempt to transfer a message asa failure. The random delay period used by a mobile station after notfinding BRI=Idle when ready to send the first burst of an access attemptor after a transmission attempt failure, is uniformly distributedbetween 0 and 200 ms with a granularity of 6.667 ms (the length of atime slot). A mobile station is not allowed to make more than Z, whereZ=(0 . . . 3), consecutive repetitions of an individual burst during anaccess attempt.

When a mobile station is in the start "random access" state, it looks atBRI information in the first occurrence of a downlink DCC slot,regardless of the sub channels of the current DCC. If the mobile stationfinds BRI=Idle, it sends the first burst unit of its message in thecorresponding sub channel uplink RACH. If BRI=Busy or Reserved, themobile station generates a random delay time. After the random delaytime has expired the mobile shall repeat the foregoing procedure. Thesearch for BRI=Idle is to be repeated a maximum of X+1 times, where X=(0. . . 9), for each transmission attempt. With the exception of anorigination, the MS still monitors its PCH during random accessprocedures.

When a mobile station is in the "start reserved access" state, it looksat BRI and PE information in all downlink slots of the current DCC,regardless of subchannels, for a slot where BRI=Reserved and a PE matchoccurs, which will be explained below. The base station (BS) can assigna reserved slot for a given mobile station MS regardless of whichsubchannel the MS may have previously used. If a mobile station findsthis slot, it sends the first unit of its message in the correspondinguplink RACH subchannel. If the mobile station does not find this slotwithin a predetermined time-out period (T), it enters the "start randomaccess" state. After the first burst of a random access has beentransmitted, the mobile reads the partial echo field in the nextdownlink slot of the current DCC that corresponds to its RACHsubchannel. If a PE match is found and R/N=1, the mobile assumes thatthe first burst of its access attempt has been correctly received by theBS and then enters either the "after last burst" state or the "moreunits" state. If the mobile does not find a PE match, it generates arandom delay time and enters the "start random access" state.

If the mobile station has additional bursts to send, it enters the "moveunits" state and then checks R/N after sending each of these bursts. Ifthe mobile station finds that R/N=Received after sending a burst, itconsiders that burst as correctly received by the BS; otherwise, itconsiders the burst as not received by the BS. If the mobile stationfinds that BRI=reserved or idle, it also considers the burst as notreceived by the BS. If more than S+1, where S=(0,1), consecutive NotBusy readings of BRI are made by the mobile, it aborts the transmissionof the message and starts another transmission attempt after a randomdelay. If a given burst is considered to be received, the mobiletransmits the next burst of the message in the next slot of thesubchannel. If a burst is found to be not received, the mobileretransmits it.

The mobile reads the SCF flags of the currently used subchannel after ithas transmitted the last burst of its current access attempt. IfR/N=Received, the MS considers the message to have been correctlytransmitted. If R/N=Not Received and BRI=Reserved or Idle, the MSconsiders the last burst as not received. If more than S+1, whereS=(0,1), consecutive combined R/N=Not Received and BRI=Not Busy readingsare made by the mobile station, it aborts the transmission of themessage and starts another transmission attempt after a random delay. IfR/N=Not Received and BRI=Busy, the mobile re-transmits the lasttransmitted burst.

The foregoing random access procedures are illustrated in FIGS. 6a-b.FIG. 6a illustrates the random access procedure for a mobile stationwhile FIG. 6b illustrates the random access procedure for a basestation. Set forth below is an itemized description of the use of thetruth table in the flow chart shown in FIG. 6a:

1. The truth table is examined at start of a contention based access atblock 10. If BRI="Reserved" or "Busy" the mobile station shall incrementthe Busy/Idle counter and then compare it to X. If BRI="Idle" the mobilestation shall set Unit₋₋ ctr to 1 and send the first burst of its accessattempt.

2. The truth table is examined after sending any given intermediateburst of an access attempt with at least one more burst pending at block20. If BRI="Idle" or "Reserved" the mobile station shall incrementStop₋₋ ctr and PB₋₋ ctr and then compare Stop₋₋ ctr to S. If BRI="Busy"and RUN="Not Received" the mobile station shall increment PB₋₋ ctr andthen compare it to Z. If BRI="Busy" and R/N="Received" the mobilestation shall increment Unit₋₋ ctr and set PB₋₋ ctr to zero.

3. The truth table is examined after sending the last burst of an accessattempt at block 30. If R/N="Received" then the mobile station shallconsider the message as successfully sent. If BRI="Reserved" andR/N="Not Received" the mobile station shall increment Stop₋₋ ctr andPB₋₋ ctr and then compare Stop₋₋ ctr to S. If BRI="Busy" and RUN="NotReceived" the mobile station shall resend the last burst of the accessattempt.

4. The truth table is examined at the start of a reservation basedaccess at block 40. If BRI="Busy" or "Idle" then the mobile stationexamines the reservation timer. If BRI="Reserved" and the PE does notmatch then the mobile station examines the reservation timer. IfBRI="Reserved" and the PE does match then the mobile station shall setUnit₋₋ ctr to 1 and send the first burst of the access attempt.

According to one embodiment of the present invention, a unique methodfor determining the status of frames sent to a mobile station isdisclosed and is illustrated in FIGS. 7a-b. The BMI (base station,mobile services switching center and internetworking function) starts anAutomatic Retransmission Request (ARQ) mode transaction by sending anARQ Mode BEGIN frame which is illustrated in FIG. 8. The ARQ Mode BEGINframe is used to start the delivery of a L3 ARCH or SMSCH message in theARQ mode. If the L3 message is too long to fit into a single ARQ ModeBEGIN frame, then the remaining L3 information is carried usingadditional ARQ Mode CONTINUE frames as necessary. If the L3 message doesfit within a single ARQ Mode BEGIN frame, it is padded with filler asnecessary. The ARQ Mode Begin frame contains a Header A and a Header Bwhich in turn contain various layer 2 overhead information. In thisexample, the Header A section has identified this frame as apoint-to-point SMS channel, however the present invention is not limitedthereto. The ARQ Mode BEGIN frame contains a Mobile station identity(MSID) and a Partial Echo (PE) field which is set to some value, forexample, the 7 least significant bits of the mobile station's IS-54Bmobile identification number (MIN). A transaction identifier (TID) fieldis included and uniquely identifies which instance of an ARQ modetransaction is being sent to a mobile station. The PE field inconjunction with the transaction identifier TID identifies thetransaction initiated by the ARQ Mode BEGIN frame and serves toassociate any subsequent ARQ Mode CONTINUE frames with the sametransaction. A L3 length indicator (L3LI) field is used by the mobilestation to calculate the number of ARQ Mode CONTINUE frames which willfollow the ARQ Mode BEGIN frame. Finally, the ARQ Mode Begin framecontains a layer 3 data field (L3DATA).

As a result of sending an ARQ Mode BEGIN frame, the BMI may wait for anacknowledgement from the mobile station since this frame containsinformation pertaining to the potential success of an ARQ Modetransaction. If the BMI decides not to wait for a mobile stationacknowledgement, it shall proceed to send an ARQ Mode CONTINUE framewhich will be described below. However if the BMI decides to wait for anacknowledgement, the BMI shall proceed as follows. For the case ofsoliciting a reservation based acknowledgement in a given downlink slot,the BMI polls the mobile station by setting the BRI flag (part of theSCF) to "Reserved" and by setting the PE field (part of the SCF) to thevalue corresponding to the target mobile station. The BMI then waits foran uplink ARQ Status frame on the same access path on which it polledthe mobile station. If an ARQ Status message is not received on thereserved access slot or is received but with an incorrect indication ofoutstanding frames, the BMI shall resend the ARQ Mode BEGIN frame up toa predetermined number of times. If the BMI does not receive a correctARQ Status message after a certain number of attempts, it shallterminate the ARQ Mode transaction. If the BMI does receive a correctARQ Status frame after a certain number of attempts, it shall proceed tosend an ARQ Mode CONTINUE frame.

The BMI sends as many ARQ Mode CONTINUE frames as necessary to completethe initiated ARQ Mode transaction. The ARQ Mode CONTINUE frame, whichis illustrated in FIG. 9, contains the Partial Echo (PE) field and atransaction identifier (TID) which together uniquely identify whichinstance of an ARQ mode transaction is being sent to the mobile station.The ARQ Mode CONTINUE frame also includes a Continuation Frame Number(FRNO) field which identifies individual continuation frames, and aportion of the L3 data (L3DATA). An ARQ Mode BEGIN frame has an implicitframe number FRNO value of 0 associated with it whereas ARQ ModeCONTINUE frames have explicit FRNO values which start at 1 and rangefrom 1 to 31. The FRNO value is incremented for each new ARQ ModeCONTINUE frame sent by the BMI to a MS in support of a specific ARQ Modetransaction.

The BMI may poll the mobile station while sending any intermediate ARQMode CONTINUE frame. If the BMI decides to issue an intermediate poll itshall proceed as follows. In a given downlink slot the BMI sets the BRIflag to "Reserved" and sets the PE field to the value corresponding tothe target mobile station. The BMI then waits for an uplink ARQ Statusmessage on the same access path on which it polled the mobile station.If an ARQ Status message is not received on the reserved access slot,the BMI resends the current ARQ Mode CONTINUE frame up to apredetermined number of times. If the BMI does not receive a correct ARQStatus frame after a certain number of attempts, the BMI terminates theARQ Mode transaction. If the BMI does receive a correct ARQ Status frameafter a certain number of attempts, it continues to send ARQ ModeCONTINUE frames that have not yet been correctly received by the mobilestation. When the BMI sends the last ARQ Mode CONTINUE frame, the BMIproceeds in a manner described below.

The BMI polls the mobile station while sending the last ARQ ModeCONTINUE frame as follows. For the case of soliciting a reservationbased acknowledgement in a given downlink slot, the BMI sets the BRIflag to "reserved" and sets the PE field to the value corresponding tothe target mobile station. The BMI then waits for an uplink ARQ Statusframe on the same access path on which it polled the mobile station. Ifan ARQ Status frame is not received on the reserved access slot, the BMIresends the current ARQ Mode CONTINUE frame up to a predetermined numberof times. If the BMI does not receive a correct ARQ Status frame after acertain number of attempts, the BMI terminates the ARQ Mode transaction.If the BMI does receive an ARQ Status frame and the FRNO MAP indicates"all correct", the ARQ Mode transaction is considered to be successfullycompleted. If the BMI does receive an ARQ Status frame and the FRNO MAPdoes not indicate "all correct", the BMI resends those ARQ Mode CONTINUEframes that have not yet been correctly received by the mobile station.The BMI resends any given ARQ Mode CONTINUE frame up to a predeterminedmaximum number of times before terminating the ARQ Mode transaction.

The operation of the SPACH ARQ mode for the mobile station will now bedescribed. After a mobile station has successfully completedtransmission of a message on the RACH (as indicated by the SCF flags),it reads up to a predetermined number of SPACH frames beginning 40 msafter successfully completing the RACH message transmission. When themobile station receives an ARQ Mode BEGIN frame with a MSID matching itsown, it proceeds as follows. The mobile station stores the TID for ARQMode transaction identification purposes. The number of pending ARQ ModeCONTINUE frames can then be calculated based on the received L3LI. TheFRNO MAP, which is a bit map, is set to RECEIVED for FRNO zero(corresponding to the ARQ Mode BEGIN) and NOT RECEIVED for all pendingARQ Mode CONTINUE frames. According to one embodiment of the presentinvention, a 1 can be used to signify that a frame has been received anda 0 can be used to signify that a frame has not been received. The FRNOMAP supports BMI transmissions up to 32 ARQ Mode frames in length (1BEGIN frame and 31 CONTINUE frames). The portion of the L3 messagecarried in L3DATA shall be stored. The mobile station then proceeds asdescribed in the next section. Of course those skilled in the art willreadily appreciate that the FRNO bit map could be expanded or reduced soas to support a greater or fewer number of frames transmitted from theBMI.

The mobile station shall read SPACH frames and respond as follows. If apoll occurs, i.e., a PE match occurs along with the BRI flag set to"Reserved", the mobile station sends an ARQ Status frame to the BMI witha FRNO MAP that indicates the reception status of all ARQ framescorrectly received to that point of the current ARQ mode transaction. Ifthe FRNO MAP indicates that an "all correct" condition exists, themobile station considers the corresponding ARQ Mode transaction to besuccessfully completed. Otherwise, if an ARQ Mode CONTINUE frame iscorrectly received for an outstanding ARQ Mode transaction (i.e., PE andTID sent in the ARQ Mode CONTINUE frame match) the mobile station storesthe L3DATA contained therein and sets the corresponding FRNO MAPposition to RECEIVED. If no ARQ Mode CONTINUE frame or poll is receivedfor a predetermined period, the mobile station can autonomously transmitan ARQ Status frame using a contention based random access. A maximumnumber of such autonomous ARQ Status frame transmissions may be sentbefore the corresponding ARQ Mode transaction is terminated.

While a particular embodiment of the present invention has beendescribed and illustrated, it should be understood that the invention isnot limited thereto since modifications may be made by persons skilledin the art. The present application contemplates any and allmodifications that fall within the spirit and scope of the underlyinginvention disclosed and claimed herein.

We claim:
 1. A method for obtaining a report on the status of framescomprising a message transmitted to a remote station, comprising thesteps of:transmitting said message to said remote station from a basestation using layer 2 frames; sending a status request to said remotestation from the base station using a layer 2 bit which is distinct fromlayer 2 bits used to carry the message; and sending a status report tosaid base station, wherein said status request contains a second layer 2bit which specifies whether the remote station should send the statusreport on a reservation basis or on a contention basis.
 2. A methodaccording to claim 1, wherein said layer 2 information is theBusy/Reserved/Idle portion of shared channel feedback information.
 3. Amethod according to claim 1, wherein said status request uses layer 2information to identify said remote station to which said status requestis being sent.
 4. A method according to claim 3, wherein said layer 2information is a partial echo (PE) portion of shared channel feedbackinformation.
 5. A base station for use in a cellular communicationsystem, comprising:means for transmitting a message to a mobile stationusing layer 2 frames; means for transmitting a status request to themobile station using a layer 2 bit which is distinct from layer 2 bitsused to carry the message; means for receiving a status report from saidmobile station in response to said status request, wherein said statusrequest contains a second layer 2 bit which specifies whether the mobilestation should send the status report on a reservation basis or on acontention basis.
 6. A base station according to claim 5, wherein saidlayer 2 information is a busy/reserved/idle portion of shared channelfeedback information.
 7. A mobile station for use in a cellularcommunication system, comprising:means for receiving a message from saidcellular system which is transmitted using layer 2 frames; means forreceiving a status request from said cellular system which istransmitted using a layer 2 bit which is distinct from layer 2 bits usedto carry the message; means for transmitting a status report to saidcellular system in response to said status request, wherein said statusrequest contains a second layer 2 bit which specifies whether the mobilestation should send the status report on a reservation basis or on acontention basis.
 8. A mobile station according to claim 7, wherein saidlayer 2 information is a busy/reserved/idle portion of shared channelfeedback information.
 9. A mobile station according to claim 7, whereinsaid mobile station transmits said status report to a base station insaid cellular system.
 10. A mobile station according to claim 7, whereinsaid status request includes layer 2 information to indicate whichmobile station is being sent a status request.
 11. A mobile stationaccording to claim 10, wherein said layer 2 information are a partialecho portion of shared channel feedback information.